Here the 1858 Wright edition substituted the plural were for the singular was, perhaps because of the immediately preceding robbers or because semantically one can refer to a band of robbers as a plurality. In this particular case, the 1874 RLDS edition did not follow the 1858 Wright edition but maintained the 1840 reading, “this band of robbers was utterly destroyed”. The critical text will retain the original was.
Summary: Maintain in Helaman 6:37 the singular was in “this band of robbers was utterly destroyed”, the reading of the earliest text.